Plasma is considered the fourth state of matter. A plasma is a collection of charged particles moving in random directions that is, on average, electrically neutral. One method of generating a plasma is to pass a current through a low-pressure gas that is flowing between two parallel conducting electrodes. Once certain parameters are met, the gas “breaks down” to form the plasma. For example, a plasma can be generated by applying a potential of several kilovolts between two parallel conducting electrodes in an inert gas atmosphere (e.g., argon) at a pressure that is in the range of about 10−1 to 10−2 Torr.
Plasma processes are widely used in many industries, such as the semiconductor manufacturing industry. For example, plasma etching is commonly used to etch substrate material and films deposited on substrates in the electronics industry. There are four basic types of plasma etching processes that are used to remove material from surfaces: sputter etching, pure chemical etching, ion energy driven etching, and ion inhibitor etching.
Plasma sputtering is a technique that is widely used for depositing films on substrates. Sputtering is the physical ejection of atoms from a target surface and is sometimes referred to as physical vapor deposition (PVD). Ions, such as argon ions, are generated and then are drawn out of the plasma, and are accelerated across a cathode dark space. The target surface has a lower potential than the region in which the plasma is formed. Therefore, the target surface attracts positive ions.
Positive ions move towards the target with a high velocity and then impact the target and cause atoms to physically dislodge or sputter from the target surface. The sputtered atoms then propagate to a substrate or other work piece where they deposit a film of sputtered target material. The plasma is replenished by electron-ion pairs formed by the collision of neutral molecules with secondary electrons generated at the target surface.
Reactive sputtering systems inject a reactive gas or mixture of reactive gases into the sputtering system. The reactive gases react with the target material either at the target surface or in the gas phase, resulting in the deposition of new compounds. The pressure of the reactive gas can be varied to control the stoichiometry of the film. Reactive sputtering is useful for forming some types of molecular thin films.
Magnetron sputtering systems use magnetic fields that are shaped to trap and concentrate secondary electrons proximate to the target surface. The magnetic fields increase the density of electrons and, therefore, increase the plasma density in a region that is proximate to the target surface. The increased plasma density increases the sputter deposition rate.